How Wave Pools Work

Introduction to How Wave Pools Work

Modern civilization is filled with artificial re-creations of nature. People decorate houses with artificial Christmas trees, wear artificial hair, play games on artificial grass and build zoo cages with artificial rocks. Scientists have even developed artificial hearts!

One of mankind's oddest re-creations of nature is the artificial ocean shore, also known as the wave pool. These popular water park attractions are sanitized, regimented revisions of the wild, organic surfs created by nature. In wave pools, the water is chlorinated, the beach is concrete and the waves arrive like clockwork, once every few minutes.

In this article, we'll peek behind the scenes of Thunder Bay, the main attraction at Wet 'n Wild Emerald Pointe in Greensboro, NC. As we'll see, it take­s a lot of sophisticated, powerful equipment to keep the waves rolling all day long. But the basic idea is very simple.

Making Waves

In the ocean, most waves are created by the wind. The rushing air pushes some water molecules together, producing a swell of water -- a disturbance in the ocean's surface -- at a particular point. These molecules push on the molecules next to them, which push on the molecules next to them and so on. In this way, the disturbance is passed along the surface of the ocean, while the individual water molecules stay in roughly the same area.

There are a number of ways to replicate this type of wave action. All you need is a basin of water and some means of creating a periodic disturbance. You could use a strong blast of air along the surface, a rotating paddle wheel (like the ones used on steamboats) or an oscillating plunger. Basically, you push on the water at one point and this energy travels outward, through the surrounding water. This is the same thing that happens when you drop a rock into a pond.

Emerald Pointe features a wave pool of this sort at Splash Island, a swimming area for younger kids. In this pool, gentle ripples spread out from the deeper end, just under a volcano structure, and wash up on the small beach.

Let's take a look at the machinery that makes this happen.

Click here to watch a video of an air-powered wave generator in action.

Man-made Waves

The machinery that creates the ripple effect within the wave pool at Emerald Pointe is very simple. In a pump room below the pool, a high-speed fan blows air into a wide metal pipe, which leads to an exhaust port at the base of the volcano. In the middle of this pipe, there is a butterfly valve, a wide disc with a swiveling metal axis rod. When the rod is swiveled one way, the disc rests horizontally in the pipe, blocking the air flow. When the rod is swiveled the other way, the disc moves to a vertical position so the air can pass.

A hydraulic piston swivels the rod back and forth at regular intervals, allowing short bursts of pressurized air to flow up to the exhaust port. These air blasts blow on the water at the base of the volcano, generating the flowing ripples.

Making small waves is fairly simple with this sort of system, but it's a lot harder to form large, surfable waves. You would need an absurdly intense blast of air or a large, awfully strong plunger. Such devices would likely be inefficient, cumbersome and dangerous, so they wouldn't make for particularly good water park attractions. Instead, water parks use water-pumping wave systems. In the next couple of sections, we'll see how this sort of wave pool works.

Click here to watch a video of the 90,000-gallon wave at Emerald Pointe's Thunder Bay.

Just Add Water

In the last section, we looked at an extremely simple wave pool. In this design, short bursts of pressurized air apply force to a relatively stable pool of water. This creates little waves, which extend outward along the surface of the water.

A larger wave pool system works differently. Instead of pushing on the water with air or a paddle, the wave machine dumps a huge volume of water into the deep end of the pool. The surge in water travels all the way to beach; the water level in the pool balances out again. Since water is fairly heavy, it pushes very hard to find its own level. If you dump more water in, you increase the size and strength of the wave.

There is a lot of powerful equipment involved in this process, but the idea is pretty simple. The wave pool has five basic parts:

A water-pumping system

A water-collection reservoir

A series of release valves at the bottom of the reservoir

A giant, slanted swimming pool

A return canal, leading from the beach area to the pumping system

A Giant Super-clean Toilet

Robert Bochenski, the maintenance manager at Emerald Pointe, compares the "Thunder Bay" type of wave pool to a giant toilet. The pump system draws water from the return canal into the collection reservoir in the same way that a toilet draws water from the water line into its tank. When the water level in the reservoir is high enough, the system opens up the release valves at the bottom of the reservoir. This is like flushing a toilet: It dumps all of the collected water into the pool, creating the wave.

In this system, the water is constantly circulating. It moves from the deep end of the pool, out to the canal, around to the pumping system and back into the deep end of the pool. The return canal is fenced off so swimmers will never be exposed to the pumping mechanism.

There are any number of ways to configure the wave-generating system in this setup. In the next section, we'll look at the specific machinery used at Emerald Pointe's Thunder Bay.

Keep it Clean

On a busy day, thousands and thousands of people will take a dip in the Thunder Bay wave pool, and even the cleanest among them leave a certain amount of dirt and oil behind. To keep the water sanitary, Emerald Pointe keeps a filtering system going 24 hours a day.

The Thunder Bay wave pool uses six massive filters, located just behind the wave generator. A powerful pump sucks water in from the canal, sends it through the filter system and shoots clean water back out. The collected dirty water is pumped to the city sewer line almost a mile away.

One of the release valves at Thunder BayClick here to watch a video of the valve opening.

Let it All Out

Thunder Bay is 84 feet (26 m) across at the deep end, and it is more than 400 feet (122 m) long. Overall, the pool holds about 2 million gallons (7.6 million L) of water, and the wave itself is 90,000 gallons (341,000 L). To make a wave this big and get it to make its way across the entire pool, you need some pretty heavy equipment.

The wave generator uses a 100-horsepower pump, a 200-horsepower pump and a 300-horsepower pump. Each pump has a motor at the top, which spins a long drive shaft. The drive shaft extends down through a pipe, to a propeller positioned 13 feet (4 m) underwater. When the drive shaft spins, it rotates the propeller, which drives water upward through the pipe (in the same way a spinning fan drives air forward).

At the base level of the pump, the pipe curves, runs horizontal for 6 feet (1.8 m), then curves up and rises another 10 feet (3 m) before finally opening into the water reservoir. In all, the pumps move the water 29 feet (9 m)! At their standard speed, the pumps drive about 40,000 gallons (150,000 L) of water per minute.

The water reservoir is broken up into eight connected chambers, each with its own release valve. The release valve has three major elements:

The valve seat - The opening that leads down to the pool

The valve plate - A wide piece of metal that fits snugly on top of the valve seat

Metal struts that pivot on a stationary steel beam attached to the reservoir walls on one end and to the valve plate on the other end

The hydraulic-cylinder piston, which is attached to a metal beam running between the two struts

Since the metal struts pivot freely on the stationary steel beam, you can swing the valve plate back and forth. The plate is fairly heavy, so it naturally will swing over the valve seat. This plugs up the reservoir so water can't escape. (A rubber gasket around the valve seat keeps the valve from leaking too much.)

When pressurized oil is directed to the hydraulic cylinder, it moves the piston with a great deal of force. This pushes out on the metal struts. The valve plate extends outward, unplugging the water reservoir. The water drops directly underneath the reservoir, into a curved passageway leading to the pool. As it rushes into the pool, the water ramps up a sort of reef in the concrete floor. This focuses the flowing water so that it moves in a good-sized, surfable wave.

The water pumps do most of the work in building up a wave, but the hydraulic system is the crucial element that actually gets it going. In the next section, we'll look at the simple but ingenious mechanism that controls this system.

Out with the Old, In with the New

A wave pool, just like any swimming pool, is constantly losing water. Some of it evaporates into the air, some of it is filtered out and a good bit is carried away in swimmers' bathing suits (and on their bodies).

To keep the water level up in Thunder Bay, Emerald Pointe has to constantly pump in new water. The park is connected to the city water line, but most of the water comes from several underground wells. The wells pump water into the wave-pool canal as well as into other feeder pools in the park. Installing the water wells required a hefty initial investment, but it meant much lower monthly water bills in the long run.

The directional control valve that operates the release gates at Thunder Bay

Oil and Water

In the last section, we saw that the wave generator's valves are opened by hydraulic pistons. As with most any hydraulic machine, these pistons are powered by a basic hydraulic pump. The pump keeps a steady stream of pressurized oil flowing into the system. Emerald Pointe actually uses vegetable oil, rather than regular machine oil, since there is some risk of the oil leaking into the wave pool.

In the wave generator, the oil flow is controlled by a directional control valve. The valve is connected to several hoses, leading to:

The hydraulic pump

A collection tank that feeds oil back into the pump

The cylinder pistons

If you were to cut open one of the piston cylinders, you would see something like this:

You can see that the piston rod that extends outside the cylinder is actually moved by a piston head inside the cylinder. There is fluid on both sides of this piston head, fed by two different hoses. If the force is greater on the blue side, the piston will move to the left, and if it is higher on the orange side, the piston will move to the right. To change the direction of force, you stop pumping oil to one side and start pumping it to the other. This sort of piston cylinder is commonly called a hydraulic ram.

The directional control valve directs the flow to these rams. It has a small spool that moves back and forth, blocking some inlet and outlet ports and opening others. In this animation, you can see the basic design of this sort of hydraulic system:

When the spool is to the left, the high-pressure oil goes to the left side of the cylinder, pushing the piston outward. When the spool is to the right, it pushes the piston back in. As the high-pressure oil pushes on one side, the low-pressure oil on the other side is forced back into the oil tank.

The spool is moved back and forth by a simple solenoid (an electromagnetic switch controlled by an electrical current). The solenoid is controlled by an electrical timer. Every two minutes, the timer activates the solenoid so it pushes the pistons out for a few seconds and then pulls them back in. This opens up all of the release valves, creating a wave.

One of the most amazing things about this control system, and the wave generator in general, is how durable it is. All summer long, the Emerald Pointe crew starts the waves at the crack of dawn and doesn't shut the system down until the park closes in the evening. That's one tough work week!